Method to increase electromigration resistance of copper using self-assembled organic thiolate monolayers

ABSTRACT

Methods and solutions for forming self assembled organic monolayers that are covalently bound to metal interfaces are presented along with a device containing a self assembled organic monolayer. Embodiments of the present invention utilize self assembled thiolate monolayers to prevent the electromigration and surface diffusion of copper atoms while minimizing the resistance of the interconnect lines. Self assembled thiolate monolayers are used to cap the copper interconnect lines and chemically hold the copper atoms at the top of the lines in place, thus preventing surface diffusion. The use of self assembled thiolate monolayers minimizes the resistance of copper interconnect lines because only a single monolayer of approximately 10 Å and 20 Å in thickness is used.

The present patent application is a divisional of Application No.10/413,919 filed Apr. 14, 2003, now U.S. Pat. No. 6,858,527.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to the field of makingreliable semiconductor devices, and in particular the prevention of theelectromigration of copper lines.

2. Discussion of Related Art

Advances in semiconductor manufacturing technology have led to thedevelopment of integrated circuits having multiple levels ofinterconnect. In such an integrated circuit, patterned conductivematerial on one interconnect level is electrically insulated frompatterned conductive material on another interconnect level by films ofmaterial such as, for example, silicon dioxide. These conductivematerials are typically a metal or metal alloy. Connections between theconductive material at the various interconnect levels are made byforming openings in the insulating layers and providing an electricallyconductive structure such that the patterned conductive material fromdifferent interconnect levels are brought into electrical contact witheach other. These electrically conductive structures are often referredto as contacts or vias.

Other advances in semiconductor manufacturing technology have lead tothe integration of millions of transistors, each capable of switching athigh speed. A consequence of incorporating so many fast switchingtransistors into an integrated circuit is an increase in powerconsumption during operation. One technique for increasing speed whilereducing power consumption is to replace the traditional aluminum andaluminum alloy interconnects found on integrated circuits with a metalsuch as copper, which offers lower electrical resistance. Those skilledin the electrical arts will appreciate that by reducing resistance,electrical signals may propagate more quickly through the interconnectpathways on an integrated circuit. Furthermore, because the resistanceof copper is significantly less than that of aluminum, thecross-sectional area of a copper interconnect line, as compared to analuminum interconnect line, may be made smaller without incurringincreased signal propagation delays based on the resistance of theinterconnect.

As device dimensions shrink, so does conductor width—leading to higherresistance and current density. Increasing current density leads to thephenomenon of electromigration. Electromigration is generally themovement of atoms in a metal interconnect in the direction of currentflow. Most metal atoms that move during electromigration are displacedat the top of an interconnect line where there is no barrier layer toprevent their displacement. This is called surface diffusion. Surfacediffusion can cause vacancies, which lead to voids and hillocks, andultimately to electromigration failure of the device.

Others have tried to solve this problem by alloying the copper lineswith another metal. One method includes the doping of the entire metalinterconnect line with metallic dopants in order to prevent movement ofthe atoms of the metal interconnect line in the direction of the currentflow. The dopants will either physically inhibit the movement of copperatoms or enlarge the copper grain size such that the diffusion path ofthe copper atoms is eliminated. However, blanket doping of the metalinterconnect layer results in an increased resistivity of theinterconnect layer, which degrades performance of the semiconductordevice. In response to this increased resistivity the portion of thecopper line that is doped has been decreased to only the outer edges orthe top of the line to prevent surface diffusion. Shunt layers have alsobeen used to prevent electromigration. Shunt layers are thinelectrically conductive layers formed around the copper lines. Shuntlayers prevent electromigration by physically inhibiting the movement ofcopper atoms. Additionally, shunt layers are several hundred angstromsthick and result in increased line to line leakage due to non-selectivedeposition. But, due to the further scaling down of devices and thenarrowing of copper interconnect lines, the resistance caused by thedoping of the outer layers of the lines and by the shunt layers hasbecome significant.

Embodiments of the invention provide processes and devices that moreeffectively reduce electromigration, in particular surface diffusion,without significantly increasing conductor resistance. These embodimentsare valuable in minimizing the electromigration of scaled down copperlines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is an illustration of a cross-sectional view of a dualdamascene structure after the dielectric layer has been etched to formboth vias and trenches.

FIG. 1 b is an illustration of a cross-sectional view of a dualdamascene structure after the vias and trenches have been filled with acopper layer.

FIG. 1 c is an illustration of a cross-sectional view of a dualdamascene structure after the copper layer has been polished.

FIG. 2 is an illustration of a cross-sectional view of the copperdamascene structure of FIG. 1 after a self-assembled organic monolayerhas been covalently bound to the copper interfaces.

FIG. 3 is an illustration of a thiolate molecule reacting with a metalsurface.

FIG. 4 is an illustration of a cross-sectional view of a copperinterface on which a self-assembled thiolate monolayer has been formed.The chemical bond between the copper and sulfur atoms is featured.

FIG. 5 is an illustration of a cross-sectional view of the copperdamascene structure of FIG. 1 c after a self-assembled organic monolayerhas been formed on the copper interfaces and a silicon based layer hasbeen formed over the thiolate monolayer.

FIG. 6 is an illustration of a cross-sectional view of a copperinterface on which a monolayer of 11-trichlorosilyl undecyl thioacetatehas been formed, one which a silicon based layer has been formed. Thechemical bonds between the copper interface, the monolayer, and thesilicon based layer are featured.

FIG. 7 is a flow chart showing a copper damascene process of forming asemiconductor device including forming an organic layer that iscovalently bound to a metal layer.

FIG. 8 is a flow chart showing a copper damascene process of forming asemiconductor device including a polishing step during which an organiclayer that is covalently bound to a metal layer is formed by a slurry.

FIG. 9 is a flow chart showing a copper damascene process of forming asemiconductor device including a cleaning step during which an organiclayer that is covalently bound to a metal layer is formed by a rinse.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Devices and methods employing thiolate layers to prevent theelectromigration of copper interconnects are described. In the followingdescription numerous specific details are set forth to provide anunderstanding of the embodiments of the present invention. It will beapparent, however, to those skilled in the art and having the benefit ofthis disclosure, that the embodiments of the present invention may bepracticed with materials and processes that vary from those specifiedhere.

Terminology

The terms chip, integrated circuit, monolithic device, semiconductordevice or component, microelectronic device or component, and similarterms and expressions, are often used interchangeably in this field. Thepresent invention is applicable to all the above as they are generallyunderstood in the field.

The terms metal line, trace, wire, conductor, signal path and signalingmedium are all related. The related terms listed above, are generallyinterchangeable, and appear in order from specific to general. In thisfield, metal lines are sometimes referred to as traces, wires, lines,interconnects or simply metal.

The terms contact and via both refer to structures for electricalconnection of conductors from different interconnect levels. These termsare sometimes used in the art to describe both an opening in aninsulator in which the structure will be completed, and the completedstructure itself. For purposes of this disclosure contact and via referto the completed structure.

The term copper interface refers to the copper surface that is exposedafter a copper layer has been planarized by chemical mechanicalpolishing. A copper interface is typically an exposed copper line or viathat will be subsequently covered with another layer to form afunctional semiconductor device.

The term self-assembled monolayer refers to a film that is formed bymolecules that will react with a surface in such a way that they line upin a uniform manner to create a homogeneous film that is only onemolecule thick. Specifically, they “self assemble” because eachself-assembling molecule forms a highly selective bond with copper andorientates itself perpendicular to the face of the copper surface.Through this reaction a uniform monolayer film is formed.

The terms thiol, thiolate, and X-alkanethiolate all refer to compoundscontaining sulfur. A thiol is a sulfur containing compound where thesulfur atom is terminated by hydrogen (X—S—H). A thiolate is a moregeneral term, referring to compounds where the sulfur is bound to anysubstituent including copper (X—S—Y). X-alkanethiolates refer tothiolates where the sulfur is bound to an organic compound that isalkane based and the alkane is terminated by a substituent X(X—(CH₂)_(n)—S—Y).

Embodiments of the Invention

Methods and solutions for forming organic layers covalently bound tometal layers are presented along with devices containing organic layerscovalently bound to metal layers. In a preferred embodiment, organicmonolayers that form covalent bonds to metal by self-assembly areutilized to prevent the electromigration and surface scattering ofcopper atoms while minimizing the resistance of the interconnect lines.Electromigration and surface diffusion is prevented because the organiclayer is covalently bound to the metal atoms in the metal interface. Thecovalent bonds will chemically hold the metal atoms in place.Additionally, in a preferred embodiment, the organic molecules in theorganic layer are relatively large and will help hold the metal atoms inplace because it is virtually impossible for metal atoms to migrate whencovalently bound to large organic molecules. The resistance of theinterconnect lines is minimized because, in a preferred embodiment, onlya single monolayer of organic material is used.

In a preferred embodiment the organic layer is a self assembled thiolatemonolayer and the metal layer is copper. Self assembled thiolatemonolayers are valuable because they can form thin (10 Å to 20 Å) layersthat will cap copper interconnect lines and chemically hold the copperatoms at the copper interfaces at the top of the lines in place, thuspreventing electromigration and surface scattering.

Copper interconnect lines are formed by way of a damascene, or inlaid,metal process. Typically a dual damascene process is used to form bothvias and trenches in a single layer. FIG. 1 a illustrates a dualdamascene structure 100 after vias 110 and trenches 120 have alreadybeen etched into dielectric layer 130. A barrier layer 140 canoptionally be formed over the patterned dielectric layer 130. FIG. 1 billustrates the dual damascene structure after the vias 110 and trenches120 have been filled with copper 150. The excess copper layer 160 isthen polished using chemical mechanical polishing (CMP), resulting inthe planarized dual damascene structure illustrated in FIG. 1 c. AfterCMP, copper interfaces 170 are exposed at the top of the copper lines.

FIG. 2 illustrates an embodiment of the present invention where anorganic layer 210 is has formed covalent bonds with the copper interface170 by self assembly to form a monolayer. In a preferred embodiment theorganic molecules are thiolates. Thiolates are sulfur containingmolecules (X—S—Y) that react with metallic interfaces and form linkagesbetween the sulfur atom and the metal surface (X—S-M). The species Y ina thiolate (X—S—Y) is a surface active agent that reacts with the copperinterface to form the bond between the sulfur of the thiolate and thecopper of the interface. The species Y can be any surface active agentincluding but not limited to H, COOH, CH₃, Cl, and F, where using Cl orF will aid in the solubility of the thiolate molecule. The reactionbetween the species Y of a thiolate and a metal interface to form anX—S-M linkage is illustrated in FIG. 3. As the thiolate molecule 310comes into close proximity with the metal surface 320 (illustrated at 3a) the sulfur atom 330 will become attracted to a metal atom 340 at themetal surface and begin to form a bond 350 with the metal atom 340, asillustrated at 3 b. Next, as illustrated at 3 c, a covalent bond 360forms between the sulfur atom 330 and the metal atom 340, breaking thebond between the sulfur atom 330 and the species Y 370 to form an X—S-Mlinkage.

In an embodiment of the present invention, as illustrated in FIG. 4,this reaction will occur between X-alkanethiolate molecules 410 and theexposed copper atoms 420 at a copper interface to form a self-assembledthiolate monolayer 430. Covalent bonds are the strongest type of bond.This chemical bond will chemically hold the copper atoms in place toprevent surface diffusion. This is in contrast to copper alloys andmetal shunt layers that merely block the movement of copper atomsbecause they do not form covalent bonds with copper. Physically blockingthe copper atoms is not as effective as the chemical bond formed bythiolates with copper. The species X may be any type of substituent, butis preferably an organic substituent. Organic substituents will notcause shorts or increased leakage for narrow lines because they arenonconductive. Additionally the species X can be readily manipulated.The species X can be selected to tailor the diffusion coefficient ofcopper at the copper interface. The larger the molecule that is attachedto the copper atoms, the less the copper atoms will be able to move. Themolecular species X can be modified to be as large or bulky as possible.It is virtually impossible for a copper atom to diffuse at roomtemperature when it is attached to a sizeable organic group because ofreduced mobility. In a preferred embodiment, these sizeable organicsubstituents will be in the form of alkanes that are part of thethiolates. These types of molecules will be referred to asX-alkanethiolates. The optimal size of the X-alkanethiolates(X—(CH₂)_(n)—S—Y) is where n=10 or 11. But n may be any number of alkane(CH₂) groups. An X-alkanethiolate where n=10 is called an undecylthiolate and an X-alkanethiolate where n=11 is called an octadecylthiolate.

An additional advantage of X-alkanethiolates, and in particular undecyland octadecyl thiolates, is that they improve the corrosion resistanceof copper films to oxidation. X-alkanethiolates are anti-corrosivebecause they form densely packed monolayer structures and form strongcovalent bonds with the copper atoms that prevent the oxidation of Cu.The anti-corrosive properties are increased when a hydrophobic (waterrepelling) substituent is chosen as the X-group for X-alkanethiolates.

Additionally, undecyl and octadecyl thiolates will form a monolayerhaving the optimal thickness. The thickness of the monolayer ispreferably between 10 Å and 40 Å and optimally between 10 Å and 20 Å. Itis valuable that the monolayer has a thickness greater than 10 Å toprotect against corrosion. Also, it is valuable that the monolayer has athickness less than 40 Å to minimize the resistance of the interconnectline.

The X-group substituent of X-alkane thiolates can also be chosen topromote adhesion between different surfaces, and in particular betweencopper interfaces and materials formed on the copper interfaces. Thetype of substituent that will work the best depends on the type ofmaterial that will be deposited on the copper interface. Typically asilicon based material is used to form an interlayer dielectric (ILD) oretch stop (ES) layer on the copper interface. Silanes are a good choicefor the X-group of X-alkanethiolates when silicon based materials areformed on the copper interface. FIG. 5 shows a covalently bound organicmonolayer 510 acting as an adhesive between copper interfaces 170 of acopper damascene structure 500 and a silicon based layer 520.

An ideal adhesion promoting thiolate is 11-trichlorosilyl undecylthioacetate. This molecule has been shown to promote adhesion betweensilicon dioxide and metals such as gold. FIG. 6 illustrates a monolayerof 11-trichlorosilyl undecyl thioacetate 610 formed on a copperinterface 170. An S—Cu bond is formed between the copper atoms 640 andthe thiolate 610. And, an Si—O—Si bond 620 is formed between an ILD oran ES (etch stop) layer 630 and the thiolate 610. Typically the siliconbased layer 630 is an etch stop layer made of SiN or SiON. The Si—O—Sichemical bond is the key to the strong adhesion between the copperinterface and the silicon based layer.

The organic layer covalently bound to a metal layer can be formed byseveral different methods. Embodiments of some exemplary methods arepresented below. In general, an organic layer covalently bound to ametal layer is formed by applying a solution containing self-assemblingorganic molecules to a metal interface. The self-assembling organicmolecules will adsorb from the solution onto the metal interface to formthe organic layer. In a preferred embodiment the organic layer is amonolayer, that is, it has a thickness of one organic molecule. To forma monolayer, the concentration of self-assembling molecules in thesolution is such that there is one self-assembling molecule for everyone metal atom to which the solution will be exposed. By using thisconcentration it will be ensured that only a monolayer of theself-assembling molecules is formed. The amount of non-conductiveorganic material used is thereby minimized, that will in turn minimizethe increase in resistance of the metal lines. Additionally, themonolayer is very thin and can be etched away before forming a via. In apreferred embodiment the self-assembling organic monolayer is athiolate.

In an embodiment, the organic layer covalently bound to a metal layer isformed after CMP (chemical mechanical polishing). FIG. 7 depicts a flowdiagram of a method embodying a typical damascene process employing anembodiment of the present invention. At block 710 a patterned dielectriclayer is formed. Typically the dielectric layer will be patterned tohave several trenches and vias using the dual damascene processdescribed above. At block 720 a metal layer is formed over the patterneddielectric layer. Then, at block 730 the metal layer is polished. Afterpolishing, the covalently bound organic layer is formed on the polishedmetal interfaces at block 740 using a solution containingself-assembling organic molecules. In a preferred embodiment theself-assembling organic molecules are thiolates. The solution can besprayed or poured onto the substrate or the substrate may be immersedinto a bath containing the solution. In an embodiment, the solutioncontaining the self-assembling organic molecules is a mixture of theself-assembling organic molecules and a solvent. The solvent is chosenbased on the specific self-assembling molecule that needs to besolvated. In an embodiment, solvents such as isooctane, chloroform,tetrahydrofuran, acetonitrile, acetone, and ethanol may be used. In analternate embodiment a gaseous solution of the self-assembling organicmolecules is used. The gaseous solution would contain a gaseous form ofthe self-assembling organic molecules. In a preferred embodiment theorganic molecules are gaseous thiolates. A carrying gas such as argonmay also be mixed with the gaseous organic molecules. The gas would beapplied to the substrate by spraying or by exposing the substrate to thegas. Optionally, an anneal is subsequently performed at block 750 on theorganic layer to better align the self-assembled organic molecules intoa uniform structure.

In an alternate embodiment, the organic layer is formed during thechemical mechanical polishing (CMP) step. This has the advantage of notadding an extra step to the processing of a semiconductor device. FIG. 8depicts a flow diagram of a method where the covalently bound organiclayer is formed during the CMP step. At block 810 a patterned dielectriclayer is formed. Next, at block 820 a copper layer is formed over thepatterned dielectric layer. The CMP step is then performed at block 830.During this CMP step an organic layer that will form covalent bonds withthe metal layer is formed on the metal layer by using a slurrycontaining self-assembling organic molecules. An exemplary slurry wouldcontain self-assembling organic molecules, an abrasive, an oxidizingagent, and a chelating buffer system. The concentration ofself-assembling organic molecules in the slurry is such that aself-assembled organic monolayer is formed. In a preferred embodimentthe self-assembling organic molecules are thiolates. Typical abrasivesinclude silica, alumina, and ceria. The oxidizing agent is typicallyhydrogen peroxide. And, the chelating buffer system is typically amixture of citric acid and potassium citrate.

In an alternate embodiment, the covalently bound organic layer is formedduring the cleaning performed after CMP. This method has the advantageof not adding an extra step to the processing of a semiconductor device.FIG. 9 depicts a flow diagram of a method where the covalently boundorganic layer is formed during cleaning. At block 910 a patterneddielectric layer is formed. Next, at block 920 a metal layer is formedover the barrier layer. The metal is then polished at block 930.Following the polishing, the copper layer is cleaned at block 940 with arinse containing self-assembling organic molecules that will formcovalent bonds with the metal interface. During the cleaning step thecovalently bound organic layer is formed. An exemplary rinse wouldcontain self-assembling organic molecules, water, and an alcohol such asisopropyl alcohol or a weak acid such as citric acid. In a preferredembodiment the self-assembling organic molecules are thiolates. Theconcentration of self-assembling organic molecules in the rinse is suchthat a self-assembled organic monolayer is formed.

The components of the solutions, slurries, and rinses presented abovemay remain in their original chemical form as they existed before theywere added to the mixture or they may combine to form chemical compoundsor ionic species different from the original components as they existedbefore they were added to the mixture.

CONCLUSION

Embodiments of the present invention provide methods and solutions forforming a semiconductor device containing an organic layer that isself-assembled and covalently bound to a metal interface. Variousembodiments of such a device are also presented. In a preferredembodiment the covalently bound organic layer is a thiolate and themetal to which it is covalently bound is copper. These embodimentsemploying covalently bound organic layers prevent the electromigrationand surface diffusion of metal lines, and in particular of copper lines.Embodiments of the present invention are valuable in minimizing theelectromigration of scaled down copper lines without significantlyincreasing the resistance of copper lines.

Other modifications from the specifically described devices, solutions,and processes will be apparent to those skilled in the art and havingthe benefit of this disclosure. Accordingly, it is intended that allsuch modifications and alterations be considered as within the spiritand scope of the invention as defined by the subjoined claims.

1. A semiconductor device comprising: a dielectric layer; a copper layerformed over the dielectric layer; and an adhesion layer formed over thecopper layer wherein the adhesion layer comprises 11-trichlorosilylundecyl thioacetate and is covalently bound to the copper layer.
 2. Thedevice of claim 1, wherein the adhesion layer is a self assembledmonolayer.
 3. The device of claim 1 further comprising a layer ofsilicon based material formed on the adhesion layer.
 4. The device ofclaim 3 wherein the adhesion layer is covalently bound to both thecopper layer and the silicon based material.
 5. A semiconductor devicecomprising: a dielectric layer having at least one trench; a copper lineformed in the at least one trench, wherein the copper line has a copperinterface; and a layer of 11-trichlorosilyl undecyl thioacetate formedon the copper interface; and a silicon dioxide layer formed above thelayer of 11-trichlorosilyl undecyl thioacetate.
 6. The device of claim 5wherein the layer of 11-trichlorosilyl undecyl thioacetate is aself-assembled monolayer.
 7. The device of claim 6 wherein the monolayerhas a thickness of between 10Å and 40 Å.